Blood pressure measurement by the volume compensation method has been conventionally developed as a method of non-invasively measuring the blood pressure easily and conveniently. The volume compensation method described in Japanese Unexamined Patent Publication No. 54-50175 (patent document 1) is as follows. That is, the artery is compressed by the cuff from ex vivo, and the volume of the artery pulsating in synchronization with the heart rate is constantly maintained constant to have the pressure (cuff pressure) for compressing the measurement site and the inner pressure of the artery of the measurement site, that is, the blood pressure at equilibrium. The blood pressure value is continuously obtained by detecting the cuff pressure when the equilibrium state is maintained.
In such volume compensation method, the arterial volume signal is fed back, and the servo control is carried out so that the arterial volume is always constant, that is, so that the arterial volume value matches the control target value (volume value when artery is in no-load state). Therefore, the compression degree to the measurement site changes according to the detected arterial volume signal during the blood pressure measurement period.
Japanese Unexamined Patent Publication No. 2001-17400 (patent document 2) describes a method of detecting rapid increase of the pressure pulse wave or the blood pressure.    Patent Document 1: Japanese Unexamined Patent Publication No. 54-50175    Patent Document 2: Japanese Unexamined Patent Publication No. 2001-17400